The present invention relates to process for the treatment of bauxite ores so as to recover alumina therefrom. The Bayer method for the recovery of alumina from bauxite ores is well known and practiced widely. Generally speaking the Bayer process involves forming a slurry of the ore in a caustic solution and digesting the caustic slurry at an elevated temperature for a period of time so as to extract the caustic soluble alumina values into solution and form an alumina or sodium aluminate enriched caustic liquor in which there is suspended various insolubles such as silicon and iron compounds. The effluent from the digestion is then treated to separate the liquor from the insolubles and the liquor treated so as to recover the alumina values contained therein.
The composition of a bauxite ore will vary widely depending on its source, with the major components generally existing as a mixture of specific mineral forms each of which has differing physical and chemical characteristics. The component of most importance is of course alumina and this is usually present as a trihydrated alumina (Al.sub.2 O.sub.3.3H.sub.2 O) known as gibbsite and/or a monohydrated alumina (Al.sub.2 O.sub.3.H.sub.2 O), that is either boehmite or diaspore. The trihydrated alumina is most desirable since it is caustic soluble at relatively low digestion temperatures and presents no serious process problems, the monohydrated alumina being more difficult to dissolve and requiring higher digestion temperatures. In the lower grade ores which are common today, ninety percent (90%) or more of the alumina may be present in the form of a monohydrated alumina.
The iron in a bauxite ore will generally be present as hematite (Fe.sub.2 O.sub.3) and goethite [(FeO)OH] and sometimes a small percentage of magnetite (Fe.sub.3 O.sub.4) will be present. Hematite is a dense material which causes no serious processing problems. Iron present in the goethite phase has a lower density due to the hydrate water but is readily dehydrated at around 150.degree.C. However, when goethite is dehydrated it is believed to leave a skeletal particle having even lower particle density due to its porous character. The dehydrated particle is amorphous in character and settles with extreme difficulty in the recovery section of the Bayer process. Many bauxite ores being commercially processed at the present time may contain 90% or more of the iron present in the goethite phase.
Also present in bauxite ores will be varying amounts of undesirable elements such as silicon, zinc, phosphorus and titanium. Silica is undesirable as it will dissolve at elevated temperatures to form a sodium silicate which in turn reacts with the dissolved alumina to form an insoluble silicate complex with the approximate molar composition of 3Na.sub.2 0.3Al.sub.2 O.sub.3.5SiO.sub.2.Na.sub.2 CO.sub.3. It can be seen that up to four moles of soda and three moles of alumina can be lost to this reaction; however, the reaction must be allowed to proceed in order to prevent contamination of the product and formation of scale on vessel walls. The zinc, phosphorus and titanium which may be present, if uncontrolled can cause product contamination as well as other problems. For example, phosphorus compounds can cause serious filtration problems.
It has been known in the prior art to add lime, or other equivalent calcium compound, to solve various problems. Several advantages can be obtained by the addition of lime. For example, lime addition will cut down on soda losses through the above-mentioned insoluble silicate complex since the lime will substitute into the complex so as to liberate some soda. Lime addition also causes formation of insoluble calcium phosphate such that phosphorus contaminates may be removed. Others have reported increased alumina recovery from a bauxite containing goethite and titanium by a process involving lime addition. Thus the use of lime in the Bayer process is well known and generally recognized to be beneficial.
One of the lower grade bauxite ores which is being mined today is one containing substantial amounts of both goethite and a monohydrated alumina. The processes known in the prior art utilizing addition of lime may be used to produce a satisfactory product from such an ore; however, since the economic practicality of a given process often depends on a relative narrow margin of production, any change which will result in an increase in production and/or decrease in process costs and problems is greatly desired. It is for this reason that research is constantly underway for improving the processing of these lower grade ores.
It is thus an object of the present invention to provide a new and useful wet caustic process of the Bayer type for recovering alumina from a bauxite containing amounts of both goethite and a monohydrated alumina. A more particular object of the present invention is to provide a new and improved wet caustic process of the Bayer type and involving the addition of lime, or its equivalent, for the processing of a bauxite containing goethite and a monohydrated alumina. Additional objects will become apparent from the following description of the present invention.